Difference w/ Assumption Method - Part 2


This week's math tips is about difference concept with assumption method - part 2. This question can be spotted in Primary 5-6 papers which carries 5 marks!

Qns: Peter Participated in a Math competition. There was a total of 35 questions. For each correct answer, 3 marks were awarded. For each wrong answer, 2 marks were deducted. For each question left blank, 1 mark was deducted. Peter did not answer 5 questions and scored a total of 70 marks. How many questions did he answer correctly?

Step 1: Remove 1 condition (the questions left blank)

35 - 5 = 30 (Questions that are correct & wrong)

Step 2: Assume all questions are correct

30 x 3 = 90

Step 3: Add back the marks that was deducted due to leaving blank

70 + 5 = 75 (if there are only 30 questions of correct & wrong)

Step 4: Minus the difference

90 − 75 = 15 (Total difference)

Step 5: Difference in 1 group

3 + 2 = 5 (Add due to one deducting and one adding marks)

Step 6: Find the number of groups

15 ÷ 5 = 3

Step 7: Number of correct questions

30 - 3 = 27

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